I see. That API makes no promises about order, so you cannot rely on the order. In any case, what order could it promise if (for example) the inks are in mixtures of Separation and DeviceN in various orders.
If you want an ordered enumeration, you will need to implement your own. It just needs a recursive scan of page resources (recursing into form XObjects, patterns, type 3 fonts).
I can speculate why your different definitions will produce different results (in one case, pick elements from an array; in the other, enumerate a dictionary, which is itself undefined). But that doesn't change how the API is expected to work.